1. Field of the Invention
This invention relates generally to a method and apparatus for detecting cracks on the surface of structural members of nuclear power equipment, steam turbines, hydraulic turbines, and the like, and more particularly to a method and apparatus for detecting highly accurately the shapes of cracks which is indispensable for understanding the development state of cracks.
2. Description of the Prior Art
A method of measuring the development state of cracks by causing a current to flow around a crack and measuring a voltage across the crack is disclosed in the article "ADVANCES IN CRACK LENGTH MEASUREMENT" (editor: C. J. Beevers) in ENGINEERING MATERIALS ADVISORY SERVICES LTD., (1982). In accordance with this prior art method, however, current supply terminals and voltage measurement terminals are fixed. Therefore, when the positions of of cracks are limited to particular positions, the method can grasp the development state of the cracks, but if the crack positions are not at the center of the voltage measurement terminals, the detection accuracy of the crack depth drops and detection of the crack shape is extremely difficult.